Iron head 11.35 CR on 91 Octane

[hotrod1968]

I’m putting together an ambitious street effort 350, Vortec head, solid roller. Since it’s for myself, I’m pushing boundaries I wouldn’t dare with someone elses engine. I’ve worked the CR out to 11.35:1. I know DCR is a somewhat of an armchair science, but I was curious to know if it bears enough real world use that I could risk the big static number. I don’t expect number crunching to be a magic ball, but I was curious to see if theory and practice align at all in this instance. I can shave the dome of the pistons (L2252AF30 1.56 compression distance, 11cc dome), but I’d leave them alone unless it seems imminent that I’ll encounter detonation.
The cam is a Crower 00427 street roller

INT/EXH - Dur @ .050” Lift: 260°/266° RR: 1.5/1.5 Gross Lift: .585”/.597” LSA: 112° RPM: 3250 to 7250 Redline: 7750

I got the following DCR/Cranking compression with an online calculator:
Int closing point 77deg ABDC
Static compression ratio of 11.35:1.
Effective stroke is 2.39 inches.
Dynamic compression ratio is 8.11:1 .
Dynamic cranking pressure is 166.49 PSI.

The heads were cc’d to 67.5 and the chambers have lots of work in them.

I’m wondering what you guys think about running that much compression on 91 Octane. I know a lot of people discount the DCR as a real world thing, so I’m curious to know how I should proceed. I want it to be on the ragged edge, but I don’t want to try something foolish based on an online calculator or theory. I keep reminding myself that’s a ton of compression especially with an iron head, but I was hoping the racy street cam would be tailored to tame the static compression some. Thanks for reading

[hotrod1968]

Neglected to mention quench, the piston is .025” in the hole, .039 gasket. Figured any thinner on the head gasket would help quench, but increase CR further into the red zone. Also, I’m curious as to why the bore of the generic off the shelf head gaskets (4.100, 4.125) aren’t closer to finished bore sizes (4.030, 4.060)

[rustbucket79]

Functionally you can make it work to some level, but you’ll never achieve what it’s capable of without E 85 or 100 octane fuel or similar. Nothing here is optimized but you can still have a blast. Slow timing curve and 30 degrees or less as a starting point.

[travis]

2 things stand out to me. 1...your quench will be in that “bad zone” between .060 and .080”, which is there area where one is more likely to detonate. 2, the L2252 pistons, along with most domed pistons, will hit the chamber on a vortec head without piston and/or chamber clearancing.

I’m no expert, but I think you could get away with 11.35-1 compression with a sharp tune, loose converter, some deep gears, and conservative driving habits...anything to reduce engine load at lower engine speeds. There could be a detonation problem though once that thing gets “up on the cam” and starts really breathing.

91 octane pump fuel varies widely in quality too

[Super_Stock]

I'd rather deck it to get more quench, and lose some of the dome...

[F-BIRD'88]

Sharpe tune means retard the timing.
Travis is right ,the piston dome will likely need more than a little carving to clear the vortec chamber.

Cut off enough of it to get the CR to 10.50:1
With the .015" shim head gasket so you have correct quench and a practical CR for the fuel.

You can pick up a good set of 1.56" CH Hyper T5 flat tops for $100 or less. Do that.

[1980RS]

My 406 I ran last year with 2 sets of Vortec heads is about what you are planning on doing and I had close to 11:1 compression. I ran 100 octane because it has a 106 lca cam, if I were to put something in with a 114 LCA I bet I could get away with 93 octane. I ran 31 to 32° timing total.

[PackardV8]

Travis is right ,the piston dome will likely need more than a little carving to clear the vortec chamber.

Cut off enough of it to get the CR to 10.50:1
With the .015" shim head gasket so you have correct quench and a practical CR for the fuel.

You can pick up a good set of 1.56" CH Hyper T5 flat tops for $100 or less. Do that.

X2 - forget the dome. Go with the flat tops and get the quench right and most problems won't occur.

[Walter R. Malik]

Keep the water temperature down ...

[CGT]

Well if you are gonna try it, you got the right cylinder head on it. I am also curious how that dome is fitting the vortec chamber. I get the try it on your own engine thing. I've done plenty of that. If it were mine I would would probably tighten the quench up and accept the gain in compression, but hell I may try it your way as well.

If the ports and chamber are good, and you keep a close eye on things....water temp like previously mentioned, I think it could be ok. Definitely not something I'd do for someone else. But under the right circumstances I would try it on something of my own.....